There are several wide angle zooms, the Pentax 12-24, and the Sigma 10-20 etc. But it seems the most compelling ultrawide images come from the DA15, not the zooms. See the 15m-contols-my-mind thread on the Pentax forum.

I was thinking about getting an ultrawide zoom, to compliment my DA15, but I wasn't overly impressed with the creative photos I've seen. Is that because you get more creativity working with a fixed focus lens then a zoom, or is it because anything wider then 15mm rectalinear is so wide, that its harder to get useful creative pictures?

I almost think the DA10-17mm fisheye photos I've seen (and taken) are more visually compelling than what I've seen from the ultrawide zooms, but the DA15 easily tops them all.